A known practice in the construction industry for insulating existing walls of buildings is to add a layer of insulation board over the existing building wall and cover the insulation layer with a cementitious material to provide a suitable exterior coating with good aesthetic and weatherability properties. In this way the insulating ability of existing buildings can be cheaply and efficiently upgraded without requiring remodeling of the interior of the building. Typically, the insulation material is either a foamed insulating material, such as a urethane foam board, or a fiber glass board containing a binder. This type of wall construction can also be used for new buildings, with the boards being attached to the studs rather than to an existing wall. The coating is typically applied in two stages--a cementitious undercoat and a non-cementitious overcoat.
This construction method has some problems requiring careful attention during construction. The boards must be fastened to the existing wall, or to the studs in new construction, and this requires a mechanical fastening system. In some cases furring strips are provided for attachment of the boards. Some means is often required to help adhere the cementitious cover layer onto the insulating board. This is often accomplished with a layer of chicken wire and furring strips. In some wall constructions using fiber glass boards, the chicken wire can be eliminated by providing a fiber glass scrim material attached to the board. This allows thin layers of coating, as thin as 1/4 inch or less.
A problem with most constructions using such thin coatings is that the holes, gouges, dimples made by mechanical fasteners, and joints must be filled prior to applying the cementitious coating. Thus, the typical coating process for these wall constructions begins by first sanding off the high spots, then pre-spotting the joints, holes and dimples in the board with a pre-spotting or undercoating material, then applying a full undercoat with the undercoating material and, finally, applying a final overcoat material. Sanding is often necessary to prevent application of different thicknesses of the undercoat. The pre-spotting step is necessary because typical undercoating material will shrink and crack upon drying in locations where the thickness varies, e.g., over a gouge, depression or seam, i.e. where the undercoat is necessarily applied in greater thicknesses. The pre-spotting step is time-consuming to apply and requires a time delay for drying before the undercoat is applied. An invention which improves the ability of the undercoat to the extent of eliminating the prespotting step, and filling low spots to be flush with the high spots would greatly improve the construction art.